Electrostatic spraying of thermosetting acrylic solution coating compositions modified to include thixotropic agent

ABSTRACT

THERMOSETTING ACRYLIC SOLUTION COATING COMPOSITIONS EXHIBITING MEWTONIAN FLOW CHARACTERISTICS ARE MODIFIED TO INCLUDE THIXOTROPIC AGENTS TO IMPART NON-NEWTONIAN FLOW CHARACTERISTICS TO THE COATING SOLUTION SO THAT THE SOLUTION WILL PERFORM BETTER WHEN ELECTROSTATICALLY SPRAYED. IN THIS WAY, AND WHEN EMPLOYING ELECTROSATIC SPRAY EQUIPMENT IN WHICH THE SOLUTION IS PUMPED TO THE ATOMIZING HEAD THROUGH A NARROW ORIFICE UN ORDER THAT SURGING WILL BE MINIMIZED WHEN THE ATOMIZNG HEAD IS RECIPROCATED, THE SPRAYED COATING DOES NOT BUBBLE AND CRATER.

United States Patent 3,796,590 ELECTROSTATIC SPRAYING OF THERMOSET- TING ACRYLIC SOLUTION COATING COM- POSITIONS MODIFIED TO INCLUDE THIXO- TROPIC AGENT Lester L. Spiller, 3939 W. 56th St., Indianapolis, Ind. 46208 No Drawing. Continuation of abandoned application Ser. No. 697,650, Jan. 15, 1968. This application Oct. 6, 1971, Ser. No. 187,152

Int. Cl. B05b 5/02; B44d N08 US. Cl. 117-934 R 11 Claims ABSTRACT OF THE DISCLOSURE This application is a continuation of my prior application Ser. No. 697,650, filed Jan. 15, 1968, and now abandoned.

The present invention relates to the electrostatic spraying of thermosetting acrylic solution coating compositions employing electrostatic atomization equipment in which the coating is pumped through a narrow orifice leading to the atomization head in order that surges of coating liquid will be minimized when the atomizing head is reciprocated.

In many types of automatic electrostatic spray equipment, the spray head is reciprocated in Order that the spray particles might be directed over all portions of the object to be coated. However, and when the atomizing head changes its direction of movement abruptly, the coating liquid being pumped to the spray head surges and this interferes with proper operation of the equipment. To minimize these surges, and the splashing and spattering of paint that go with such non-uniform paint supply, the feed to the atomizing bell or head is formed to include a narrow orifice through which the coating liquid must pass shortly before it reaches the atomizing surface. In normal operation, the coating liquid is unable to surge through the narrow orifice and the aforementioned difficulties incident to reciprocation of the atomizing head are eliminated. Unfortunately, and when this expedient is utilized with thermosetting acrylic solution coating compositions, unexpected further difliculties arise in that the coatings which are deposited tend to bubble causing cratering and other surface defects.

It was at first thought that this bubbling difiiculty might be combatted by appropriately adjusting the solvent balance of the coating solution and considerable efforts were made along this line without significant success. In this regard, thermosetting acrylic coating solutions employ relatively low molecular weight hydroxy-functional addition copolymers which are highly soluble in organic solvents with the ultimate properties of the copolymer being built by means of the thermosetting cure which takes place after the solution has been deposited upon the base and baked to form the final coating. As a result of this high solubility, the coating solutions are formulated to include a large proportion of resin solids, e.g., 40% by weight or higher up to about 60-65% by Weight, and usually, at least 50% by weight, and these concentrated coating solu- 3,796,590 Patented Mar. 12, 1974 tions nonetheless possess the low viscosities appropriate for spray application, e.g., a viscosity in a No. 2 Zahn Cup of from 15-35 seconds, preferably from 16-25 seconds. These concentrated coating solutions exhibit considerable increase in viscosity upon a relatively small loss of solvent as may occur as an incident to spray application, and this may partially account for the difficulties which have been encountered and also for the fact that compensation for these difliculties by solvent balance has proven so difiicult. Nonetheless, the nature of the difficulty is not fully understood at this time.

Accordingly, the problem of the invention is restricted to thermosetting acrylic solution coating compositions which comprise a relatively low molecular weight solution addition copolymer containing at least 5% by weight of copolymerized hydroxy functional monomer in a heatcurable combination in solution in an organic solvent with the addition copolymer constituting at least 60% of the weight of total resin solids and with the total resin solids content of the solution coating composition being at least 25% by weight to provide a solution viscosity of from 10-35 seconds, preferably from 15-25 seconds, measured in a No. 2 Zahn Cup at room temperature. Such coating solutions frequently exhibit Newtonian flow characteristics, e.g. viscosity becames largely independent of shear.

In the invention, and to enable the coating solutions described above to be pumped through a narrow orifice and then electrostatically sprayed without encountering surface defects in the coatings which are formed, a small proportion of a thixotropic agent, e.g., from 0.5-8% by weight of resin solids, preferably from 2-6% byweight of resin solids, is added in order to overcome the tendency of the solution coating composition to act as a Newtonian fluid. It has been found that when non-Newtonian characteristics are imparted to the coating solution by the thixotropic agent, the tendency to bubble and crater after being forced through a narrow orifice is overcome.

To illustrate what is meant by Newtonian and non- Newtonian flow, specimens of the coating solution under consideration can be tested in a Brookfield viscometer. By a Newtonian fluid is meant one which does not change its viscosity more than 10%, comparing the viscosity in centipoises at two revolutions per minute with the viscosity in centipoises at 20 revolutions per minute. Significant non-Newtonian flow involves a reduction in viscosity of at least 30% on the basis noted hereinbefore.

The thermosetting acrylic systems under consideration are primarily based upon a solution addition copolymer containing at least 5% by weight of copolymerized hydroxy-functional monomer in a heat-curable combination. Preferably, from 8-25% of hydroxy-functional monomer is present in the solution addition copolymer. The preferred monomers are methylolated carboxylic acid amides, especially acrylamide and methacrylamide, though other alkylolating agents can be used. The alkylol, e.g., methylol groups can be etherified with an alcohol, if desired, or etherifying agents can be omitted. In order to provide effective thermosetting action, functional groups reactive with the alkylol amide groups are desirably present and these may be incorporated in the copolymer, e.g., by copolymerizing therein non-nitrogenous hydroxy-functional monomers or proportions of acidfunctional monomers or epoxy-functional monomers. On the other hand, these same reactive groups can be utilized in external resinous materials in order to provide the cure which is desired. Thus, alkyd resins and especially hydroxy-functional resins, polyepoxides and aminoplast resins can be used alone or in any desired combination in order to provide a reactive curing system. However, and in order that the problem of the invention exist, e.g., that the thermosetting acrylic solution coating composition possess Newtonian flow characteristics, any external resins used to provide cure would probably have to be present in only minor amount.

It is also possible to employ non-nitrogenous hydroxyfunctional monomers in order to provide the basis for an effective thermosetting cure. Thus, hydroxy-functional monomers such as hydroxypropyl methacrylate and the like can be utilized to provide an hydroxy-functional copolymer which may be devoid of methylolated amides. These can participate in a thermosetting cure through the utilization of aminoplast resins which may, again, be combined with hydroxy-functional alkyd resins and/or polyepoxides. In either event, e.g., whether the hydroxy group is carried as part of a N-methylol group or not, the copolymer is formed by addition copolymerization in organic solvent in order to form a low molecular weight, highly soluble copolymer which is utilized in a thermosetting system so that the desired properties are acquired by cure with the copolymer constituting the bulk of the resin solids of the solution coating composition.

The present invention is largely independent of the selection of organic solvents. These, as is known, are frequently aromatic hydrocarbon solvents such as xylene and toluene either alone or in combination with alcohols such as butanol or 2-ethoxy ethanol and the like. Proportions of diacetone alcohol, methyl ethyl ketone, butyl acetate and the like may also be present.

It is desired to point out that the low molecular weight reactive copolymers which are utilized in the invention include a significant portion of polar groups and are very extensively dissolved so that the solution coating compositions of the invention, prior to modification, possess ample electrical conductivity for electrostatic atomization. This point is stressed because, with such polar materials, there would be no need to utilize thixotropic agents in order to increase the conductivity of the system. In other words, the coating solutions of the invention atomize excellently without modification despite their Newtonian flow characteristics and require no alteraton for such purpose, unlike the situation Which prevailed in my prior US. Pat. 3,112,216.

The specific nature of the thixotropic agent appears to be of no concern whatsoever since any agent which effectively induces lowered viscosity with shear has been found to be effective in the invention in order to eliminate the bubbling and cratering which has been described hereinbefore. Of course, one must use sufiicient thixotropic agent to induce the non-Newtonian flow characteristics which are desired and one must further avoid utilizing such a large proportion of the agent as to interfere with the film properties which are desired. These factors will account for the proportions of thixotropic agent which have been noted hereinbefore.

Various agents are known for inducing desired thixotropy in an organic liquid medium. Perhaps best known among the thixotropic agents are those which are reaction products of an acidic clay with a basic organic compound.

Preferred acidic clays are those exhibiting comparatively high base-exchange properties. These include the montmorillonites, e.g., sodium, potassium, lithium and other bentonites such as Wyoming bentonite, magnesium bentonite and saponite. Other clays are montronite, attapulgite, illite, zeolites and fullers earths.

The base-exchange capacities of the various clays enumerated run from about 15 to about 100 milliequivalents of exchangeable ion per 100 grams of clay.

The basic materials which are reacted with the acidic clays are preferably strongly basic and include quaternary ammonium hydroxides such as dimethyl dioctadecyl ammonium hydroxide, amines such as octadecyl amine and oxime compounds such as ethyl methyl ketoxime, acetoxide, Z-butanone oxime, trimethoxyboroxime, dimethyl glyoxime and alpha-piccoline. The primary amines are preferred although the secondary and tertiary amine may be used. Amides such as octadecyl amide may also be employed. Polyamines such as tallow-aliphatic-diamine and polyamides such as the reaction product of dimerlc fatty acids with aliphatic diamines are also useable.

Additionally, aliphatic, cyclic, aromatic and heterocychc amines and polyamines as well as quaternary ammonium compounds may be used for reaction with the acidic clays. These are illustrated by octadecylamine, cyclohexylamlne, octyl phenyl amine, pyrrole, tetraethylene pentamine and octadecyl ammonium chloride.

Compounds having a molecular weight in excess of 300 are preferred.

Additionally, the organic base which may be reacted with the acidic clay may be constituted by onium base compounds which are isologs of ammonium. Thus, phosphonium, arsonium, stibonium, oxonium, sulfonium, selenconium, stannonium and iodonium compounds such as base salts, illustrated by octylphosphonium iodide and free salts illustrated by octylphosphonium may be reacted with the acidic clays to form thickening agents which may be used in accordance with the invention.

The above acidic clay reaction products will be illustrated by dimethyl dioctadecyl ammonium bentonite.

Most of the coating solutions which are used in the invention will include at least a small proportion of polar solvent such as butanol, but if the coating solution does not include any polar solvent, then a small proportion can be added in order that the thixotropic agent can better exert its intended thickening action.

While the treated acidic clays referred to hereinbefore are particularly appropriate thixotropic agents, the invention is not restricted thereto and other known thickening agents such as hydrogenated castor wax micronized to an average particle size of 12-15 microns by jet milling may be utilized.

The thixotropic agent can be incorporated into the paint in various ways but it is most convenient to do so before the grinding procedure. Many of the thixotropic agents are powders and these are desirably swollen in a suitable solvent and the pre-swollen agent is then stirred into the paint in which it disperses in a matter of minutes.

As a matter of interest, it is usual for the orifice leading to the atomizing bell to include a narrow orifice having an opening 0.008". With such an orifice, thermosetting acrylic solution coating compositions from several commercial sources exhibited bubbling and cratering when used and all of those which bubbled and cratered were found to possess substantially Newtonian flow characteristics. In every instance, utilization of a small proportion of thixotropic agent suificient to significantly reduce viscosity with shear overcame the bubblng and cratering problem.

It is desired to point out that the invention is in considerable measure based on the finding that Newtonian flow is responsible for the problem of bubbling and cratering. While the thermosetting acrylic coating solutions tend to possess Newtonian flow characteristics, it will be appreciated that these characteristics are not necessarily maintained in the final coating solution which is formulated. This is because when the clear coating solution is pigmented, the pigmentation of the soluton can alter its characteristics. The thermosetting acrylic solution coating compositions under consideration are frequently utilized in the production of gloss coatings and, in such environments, pigmentation is minimized in order to maintain the high gloss which is desired. Thus, and on a weight basis, the ratio of pigment to binder is usually less than 1:1, frequently less than 0.5: l, and sometimes is less than 0.311. As will be preceived, any tendency of the pigment to modify the non-Newtonian flow characteristics of the solution is largely negated by the small proportion of pigment which is utilized.

In the examples which follow, all parts are by weight.

EXAMPLE 1 39 parts of styrene, 44 parts of ethyl acrylate, 15 parts of acrylamide, 2 parts of acrylic acid, 1 part of cumene hydroperoxide and 1 part of tertiary dodecyl mercaptan are mixed with @100 parts of butanol and refluxed for 2 hours after which an additional 0.5 part of cumene hydroperoxide is added and reflux is continued for 2 additional hours. To the solution copolymer product are added 12.6 parts of formaldehyde (40% concentration in butanol) together with 0.33 part of maleic anhydride. The mixture is refluxed for 3 hours at which point half of the initially employed butanol is removed by distillation and replaced by an equal amount of xylene. To the final solution is added 25% by weight, based on resin solids, of an alkyd resin formed by the copolyesterification of 36 parts coconut oil fatty acids, 43.3 parts of phthalic anhydride, 27.5 parts of glycerin, and 2.7 parts of tertiary butyl benzoic acid. The solution so-obtained, upon ball milling with rutile titanium dioxide to provide a pigment to binder ratio of .29:1 and thinning with an equi-weight mixture of toluene and 2-butoxy diethylene glycol to a No. 2 Zahn Cup viscosity of 20 seconds possesses Newtonian flow. When this solution is pumped through an 0.008" orifice and electrostatically sprayed, the coatings bubble and crater undesirably. Upon the addition of 3.5% by weight, based on resin solids, of dimethyl dioctadecyl ammonium bentonite, the coatings exhibited reduced viscosity with shear (non-Newtonian flow) and the bubbling and cratering problem in the deposited coating disappeared. The bentonite compound is pebble milled with four times its weight of toluene to, provide a thick paste in which the bentonite compound is swollen and is milled into the coating solution as a paste to ease the mixing burden.

In place of the bentonite compound referred to above, various commercial thickening agents ,such as Bentone 34 and Bentone 38 (National Lead Co.) Baker M-P-A and Baker Thixin-R, and Nuodex Nuvis 2 are all useful in the same amount and may be added in the same way.

EXAMPLE 2 Example 1 is repeated utilizing by weight of diglycidyl ether of bisphenol A having an epoxide equivalent weight of 175 in place of the 25 by weight of alkyd resin used in Example 1. Substantially the same results are obtained and the thickening agent converted the Newtonian flow to non-Newtonian flow in order to eliminate the bubbling and cratering problem.

EXAMPLE 3 200 grams of xylol, 100 grams of butanol and 75 grams of 40% formaldehyde solution in butanol are charged to a reactor equipped with an agitator, condenser, Dean- Stark trap, thermometer, and nitrogen inlet and are heated to reflux. 130 grams of acrylamide are dissolved in 280 grams of 2-butoxy ethanol and 160 grams of n-butanol and this solution is premixed with 200 grams of 40% The unsaturated polyester resin is prepared by charging 270 parts of dehydrated castor oil, 382 parts of tall 011 fatty acids, 420 parts of glycerin, and 318 parts of isophthalic acid to a reactor where the mixture is heated to 420 F. and maintained at this temperature until a clear bead is formed when a. sample withdrawn from the reactor is cooled to room temperature. The mixture is then cooled to 350 F. and 100 parts of adipic acid, 130 parts of a polyol formed by reacting 1 mole of bisphenol A with 2 moles of ethylene oxide, and 212 parts of isophthalic acid are added and the mixture reheated to 430 F. until the acid value is reduced to 10. The product is then dissolved in n-butanol to 80% solids.

over 2 /2 hours, maintaining the temperature at reflux while removing water. The resin solution so-produced is pigmented with rutile titanium dioxide to a pigment to hinder ratio of .85 to '1 and is thinner to a No. 2 Zahn Cup viscosity of 20 seconds, and exhibited Newtonian flow. When the coating so-formulated is pumped through an 0.008" orifice and electrostatically sprayed, the deposited coatings tend to bubble and crater and these tendencies are overcome by incorporating 3.5% by weight, based on resin solids, of the thickening agents set forth in Example 1.

As a matter of interest, it is generally preferred to use as little of the thickening agent as is possible in order to induce adequate thixotropy. This is because the presence of the thickening agent can introduce a flattening tendency and it is desired in most instances to maximize film gloss.

EXAMPLE 4 263.5 grams of methyl methacrylate, 263.5 grams of styrene, 108.1 grams of ethyl acrylate, 80.3 grams of phydroxypropyl methacrylate, 14.6 grams of itaconic acid, 10.95 grams of benzoyl peroxide and 164.0 grams of 2- methoxy ethanol are charged over a 2 hour period to a reactor containing 566 grams of xylene maintained at 106 C. with stirring under a nitrogen atmosphere. After 2 hours, 0.73 gram of additional benzoyl peroxide is added and the temperature maintained at 110 C. for an additional 1 /2 hours whereupon 0.73 gram of benzoyl peroxide is added and the reaction continued for an additional 8 hours to provide a solution of hydroxy functional copolymer. 200 parts of rutile titanium dioxide are milled with 200 parts of the copolymer solution referred to above and then there is mixed in 110.8 parts of additional copolymer solution together with 66.6 parts of diglycidyl ether of bisphenol A having an epoxide equivalent weight of 175 (as a 50% solution in 2-ethoxyethy1 acetate) and 66.6 parts of butylated hexamethylol melamine (50% solution in butanol). Additional solvent is then added, namely, 195.8 parts of xylene and 97.9 parts of 2-ethoxy ethyl acetate and the solution is adjusted to a viscosity of 20 seconds in a No. 2 Zahn Cup by adding a mixture of xylene and 2-ethoxy ethyl acetate (6923.1 weight ratio). The pigment to binder ratio of the pigmented solution is substantially 0.921. The solution exhibits Newtonian flow and, when electrostatically sprayed using an 0.008" orifice, tends to bubble and crater. The addition of 3.5 by weight, based on resin solids of thickening agent as in Example 1, overcomes the bubbling and cratering problem.

In all of the foregoing examples electrostatic spray was effected utilizing a 0.008" gap probe bell which is rotated 900 r.p.m. with the bell being charged to kv. and maintained at a distance of 9 inches from a grounded metal surface to be coated. The coating solutions are pumped through the gap to the bell at a delivery rate of cos/min.

The invention is not to be construed by any abstract of disclosure, but its features are instead characterized in the description given hereinbefore and is defined in the claims which follow.

I claim:

1. A method of eliminating bubbling and cratering in a deposited film when a thermosetting acrylic solution coating composition possessing Newtonian flow characteristics is forced through a narrow orifice shortly before it reaches the atomizing head of an electrostatic spray device, said composition comprising organic solvent having dissolved therein a low molecular weight solution addition copolymer containing at least 5% by weight of copolymerized monoethylenic hydroxy functional monomer selected from the group consisting of methylolated carboxylic acid amides and ethers thereof, and non-nitrogenous hydroxy functional monomers, the latter in a heat-curable combination including organic solvent-soluble heat-hardening aminoplast resin, with said copolymer constituting at least 60% of the weight of total resin solids in said coating composition, said composition having a resin solids content of at least 25% by weight, comprising incorporating in said coating composition from 0.5 to 8% by weight of resin solids in said coating composition of thixotropic agent which is a reaction product of an acidic clay with a basic organic compound to provide a coating composition having a solution viscosity of from 15-35 seconds in a No. 2 Zahn Cup at room temperature and possessing non- Newtonian flow characteristics, feeding said composition with the thixotropic agent therein through a narrow orifice to the atomizing head of the electrostatic spray device, and electrostatically charging and depositing particles atomized from the coating composition on a surface to 'be coated.

2. A method as recited in claim 1 in which said heatcurable combination includes organic polyepoxide.

3. A method as recited in claim 1 in which said acidic clay has a base exchange capacity of from 15-100 milliequivalents of exchangeable ion per 100 grams of clay.

4. A method as recited in claim 3 in which said clay is reacted with an organic quaternary ammonium base.

5. A method as recited in claim 1 in which said organic solvent comprises a small proportion of polar solvent.

6. A method as recited in claim 5 in which said organic solvent is a mixture comprising aromatic hydrocarbon solvents and alcohols.

7. A method as recited in claim 1 in which said copolymer contains methylolated acrylamide in an amount of from 8-25% by weight and the coating composition has a No. 2 Zahn Cup viscosity at room temperature of from 16-25 seconds.

8. A method as recited in claim 1 in which said composition is pigmented, the pigment to binder ratio being less than 1:1.

9. A method as recited in claim 8 in which said pigment to binder ratio in less than 0.5 :1.

10. A method as recited in claim 9 in which said pigment is predominantly titanium dioxide.

11. A method of eliminating bubbling and cratering in a deposited film when a thermosetting acrylic solution coating composition possessing Newtonian flow characteristics is forced through a narrow orifice shortly before it reaches the rotating atomizing head of an electrostatic spray device, said composition comprising organic solvent having dissolved therein a low molecular weight solution addition copolymer containing at least 5% by weight of copolymerized hydroxy functional monomer selected from the group consisting of methylolated carboxylic acid amides and ethers thereof, and non-nitrogenous hydroxy functional monomers, the latter in a heat-curable combination including organic solvent-soluble heat-hardening aminoplast resin, with said copolymer constituting at least of the weight of total resin solids in said coating composition, said composition having a resin solids content of at least 25% by weight, comprising incorporating in said coating composition from 0.5 to 8% by weight of resin solids in said coating composition of thixotropic agent which is a reaction product of an acidic clay with a basic organic compound to provide a coating composition having a solution viscosity of from 15-35 seconds in a No. 2 Zahn Cup at room temperature and possessing non-Newtonian flow characteristics, pumping said composition with the thixotropic agent therein through a narrow orifice to the rotating atomizing head, and maintaining an electrostatic field between said composition with thixotropic agent on the rotating atomizing head and a surface to be coated for electrically charging and depositing atomized particles of said coating composition on said surface.

References Cited UNITED STATES PATENTS 1/1964 Hines l1793.4 R 3,414,635 12/ 1968 Edwards et al. 260851 3,210,273 10/1965 Taulli 260-851 3,399,152 8/1968 Jamrog et a1. 260-335 3,112,217 11/1963 Spiller 1l793.4 R 3,112,216 11/1963 Spiller 11793.4 R

OTHER REFERENCES WILLIAM D. MARTIN, Primary Examiner M. SOFOCLEOUS, Assistant Examiner US. Cl. X.R.

117104 R, Dig. 5; 260-33.6 U, 851

"UNITED sTATEs PATENT OFFICE CERTIFICATE OF CORRECTION Patent 3,79 6,59Q March 12, 197

])n ted InVent r( Lo It is certified that error appears in the aboye-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2,. line 25, "becames" should be becomes Column 3, line 39, "alteraton" should be alteration Column 5, line 7h, after "tertiary amines" insert as well as hydroxyl amines such as octadecyl hydroxyl amine Column 6, line "thinner" should be thinned Column 7, line 37, "in" should be is Signed and sealed this 5th day of november 1974.

(SEAL) Attest:

McCOY M. GIBSON JR. C. MARSHALL DANN Arresting Officer Commissioner of Patents F ORM P0-1050HO-69) USCOMM-DC 60376-P69 u.s. GOVERNMENT mmmus OFFICE: 930 

